This invention relates to an improved solid state sensing device for selectively detecting the presence of a given constituent within an atmosphere.
A solid state sensor having the ability of detecting the presence of many undesirable gases and vapors within an atmosphere is disclosed by Loh in U.S. Pat. No. 3,751,968. A solid state element, that contains alkali metal ions which readily accept negative ions of the subject gases and vapors, is brought into reactive contact therewith. The element is specially prepared to create an outer layer along its boundaries that is deplete of ions. The conductivity of the element at ambient conditions is negligible. Heating the element, however, in the presence of one or more of the reactive gases and vapors causes ions to flow across the depletion boundary and increase the conductivity of the element. Electrical circuit means are provided for detecting an increase in the conductivity of the element and generating a signal indicative of the presence of a reactive constituent in the test atmosphere.
The Loh type device has proven to be an extremely useful tool for sensing the presence or absence of a halogen gas within a specific atmosphere. Special applications include leak detection in refrigeration equipment and the presence of potentially dangerous gases within an operating room or the like. However, many test atmospheres contain more than one constituent that can react with the sensing element and, as a result, unwanted interference signals are sometimes generated that make it difficult to discern the presence of a single gas or vapor of immediate interest. Water vapor, which is ordinarily present in air, has the ability to trigger the sensor and has proven to be troublesome when air sampling is required. Because most prior art sensors of this type lack discrimination, it heretofore has been the practice in many applications to isolate the sensing element and pretreat the atmosphere prior to sampling in order to avoid erroneous triggering of the device. This, of course, increases the complexity and cost of the equipment and seriously limits its applicability.
It should be further noted that fabrication of the Loh sensor involves the building up of the solid state element around a central electrode contained within an electrical heating element. Because of the size of the components involved and the nature of the materials employed, this has proven to be a tedious and time consuming task requiring mostly hand labor. As a consequence, the volume rate of unit production is relatively low and the cost per unit relatively high.